Prior n-bit digital-to-analog (D/A) or analog-to-digital (A/D) converters typically employ a so-called voltage ladder having 2.sup.n equally spaced taps. Each of these 2.sup.n taps is used as input to one of 2.sup.n dual-input voltage comparators. The second input of each of the 2.sup.n comparators is a voltage determined by the input of the converter. One prior method of constructing the voltage ladder in integrated circuits is to use a high resistivity film, tapped at such points along its length as to provide the requisite voltages.
One prior problem of the prior resistive voltages ladders is that the procedures for depositing and etching the high resistivity films are not common to most MOS fabrication processes. Thus it is typically expensive to introduce A/D or D/A converters into circuits fabricated on most MOS process lines. Also, even when available, these resistive ladders are typically physically large and thus expensive to fabricate.
Another prior method of constructing the voltage ladder is to use an array of metal-oxide-semiconductor (MOS) transistors, each transistor of the array having a different threshold voltage corresponding to one of the desired voltages of the voltage ladder. The threshold voltage of each of the MOS transistors in a D/A converter is approximately equivalent to one of the analog voltages to which a digital signal can be converted. The input digital signal selects the associative one of the MOS transistors of the D/A converter to conduct, thereby forming an analog voltage output which is equivalent to the threshold voltage of the selected MOS transistor. Each of the MOS transistors in an A/D converter acts as a comparator to compare an input analog voltage with its threshold voltage. When the input analog voltage is applied to the gate of each of the MOS transistors of the A/D converter, those transistors having threshold voltages less than the input analog voltage are turned on while those transistors having threshold voltages greater than the input analog voltage are turned off.
The unique threshold voltage in each of the MOS transistors is typically set by semiconductor processing. As is known, the threshold voltage of a MOS transistor is determined during fabrication by the semiconductor processing. Various prior art ways are known that can process the MOS transistors to different threshold voltages.
One prior problem of the prior MOS voltage ladders is the high cost of fabricating transistors having 2.sup.n different threshold voltages. Typically this would require many additional photomasking steps, or specialized focused ion beam implanters. Historically, these costs have made it impractical to use such methods except for research purposes. Moreover, once the D/A or A/D converter having the MOS transistors is fabricated on a silicon substrate, the threshold voltages of these MOS transistors are irreversibly determined and cannot be altered. Frequently, a change in circuit design or application requirements may require a change in the conversion range of a prior D/A or A/D converter in the circuit.
Another disadvantage is that the formation of the unique threshold voltage for each of the MOS transistors by the semiconductor processing is typically very difficult and costly. First, because of the process variation, it is typically very difficult to make the threshold voltage accurately. Second, the semiconductor processing to set the unique threshold voltage in each of the MOS transistors is typically very complicated and typically cannot be used during mass production.